1. Field of the Invention
The present invention relates to an optical memory device that can record information in an optical recording medium and reproduce recorded information from the optical recording medium. Examples of the optical recording medium include compact discs (CDs) and digital video discs (DVDs).
A claim of priority is made to Japanese patent application no. 2004-203602, filed Jul. 9, 2004, the contents of which are incorporated herein by reference.
2. Description of the Related Art
A variety of writable optical recording mediums are commercially available which allow users to record large amounts of information. Examples include write once read memory (WORM) type optical disks, such as CD-R discs, and rewritable type optical disks, such as CD-RW discs. More recently, high-capacity digital video discs (e.g., DVD-R, DVD-RW and DVD-RAM) have been introduced as the result of the use of shorter wavelength laser beams, decreased spot diameters, and thin substrates. Generally, data is recorded in the form of non-reflective marks along a spiral track of the optical discs. Herein, the term “mark” refers to the pit, spot or region along the track in which data of a given logic value (e.g., “1”) is recorded along the track. For example, the marks may be formed as non-translucent dye regions in the case of CD-R discs, or as amorphous regions in the case of CD-RW discs. The spaces located between the marks along the spiral track are often referred to as “lands”. The lands are typically formed as translucent dye regions in the case of CD-R discs, and as polycrystalline regions in the case of CD-RW discs.
In order to avoid extended gaps (which can cause tracking problems) where there are no marks in the data track of a compact disc, data is typically encoded using “Eight to Fourteen Modulation” (EFM). In EFM, 8-bit bytes are converted to 14 bits, with at least some of the 14 bits being logic “1” (marks). However, variations in physical characteristics among optical discs can result in improper recording of the EFM signal. That is, mark formation failures can occur for a variety of reasons, such as variations in the composition of the dye recording layer of the optical disk, and heat accumulation of the recording media coupled with an insufficient cooling rate. In other words, the writer parameters that might be best suited for one optical disc may not be ideal for another optical disc.
As such, in an effort to improve write accuracy, a reference write pattern is used to determine and set optimal or preferred write parameters of an optical disk to be recorded in advance of a recording operation. Herein, the set write parameters associated with an optical disc are referred to as a “write strategy” of the optical disc. It is known that this write strategy is closely related with a number of factors, including recording rate, dye material, phase-change material, dye film thickness, track configuration, and so on.
The write strategy generally defines or sets a number of variable write conditions of the optical disc. For example, the mark-to-land ratio may be varied, an additional pulse may be added to the front end of a write pulse, the rise or fall position of the pulse may be altered, a recording pulse may be converted to a multi-pulse format, and so on.
By varying the mark-to-land ratio, it becomes possible to provide a favorable shape for the front end and rear end of the mark, which is generated by irradiating a short pulse onto the optical disk at a high recording power, by shortening the length of the pit during a low speed recording.
Adding an additive pulse to the front end of a recording pulse enhances the shape of the front end, which is formed by additively applying a recording power to the front end of a pit that is difficult to deform by heat due to irradiation of a laser.
Changing the rise or fall position of a pulse by a combination of the mark and land can make uniform the length of the mark and land, which are formed by changing the position of the front end of the land according to the length of a rear land because the heat of any marks influences a next mark, the position of any land is changed or heat of the recording mark is transferred to a front according to the length of a front land, a rear position of a mark is changed or heat of a rear mark is transferred to a rear land to influence the rear land according to the length of the recording mark.
The method of converting a recording pulse into a multi-pulse is mainly used for a phase change disk, such as CD-RW, or DVD. When recording information in a phase change disk using a successive pulse, heat generated during the recording operation erases the front end of the recorded information. To solve the above erase problem, a multi-pulse having a cooling period between marks is used.
Since there exist in the market too many kinds of optical disks so that the manufacturers cannot grasp completely, it is impossible to prepare a proper write strategy in advance for all optical disks in the market.
Owing to the above problem, a method of recording information while varying a write strategy into two or more tracks in an optical disk area and selecting a write strategy of which reproduction jitter is minimized (e.g., Japanese Patent Publication No. 2000-30254), or a method of recording information using a specific record pattern and obtaining a combination of mark and land by which a jitter value or a deviation value is minimized (e.g., Japanese Patent Publication No. 2003-30837), is proposed.
However, in the method disclosed in Japanese Patent Publication No. 2000-30254, a finally selected write strategy is only the most favorable write strategy among the set write strategies and is not the optimal write strategy for an optical disk in use. In addition, the above method is problematic in that tracks are used as a record area for a test.
Also, since the method disclosed in Japanese Patent Publication No. 2003-30837 uses a specific write pattern, an influence of mark or land other than the case of changing a particular mark or land is not sufficiently considered. Accordingly, it is very difficult to set an optimal write strategy for an optical disk in use.
Further, according to the standard of DVD-R, variable combinations of mark and space include only three methods of 3T, 4T, and 5-14T. Meanwhile, since the variable range of each parameter is limited to a narrow range of −0.1T to +0.05T, a method, which approaches deviation to zero while the mark and space are put aside, has a limitation.